1. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Laser-based precision spectroscopy and the optical frequency comb technique 1 Dr. Björn Hessmo Physikalisches Institut, Universität Heidelberg 1 Alternatively: Why did Hänsch win the Noble prize?

2. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2005 with one half to Roy J. Glauber Harvard University, Cambridge, MA, USA "for his contribution to the quantum theory of optical coherence“ and one half jointly to John L. Hall JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO, USA and Theodor W. Hänsch Max-Planck-Institut für Quantenoptik, Garching and Ludwig-Maximilians-Universität, Munich, Germany "for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique" What is “laser-based precision spectroscopy”? What is the “optical frequency comb technique”? What is “laser-based precision spectroscopy”? What is the “optical frequency comb technique”? The Nobel prize in physics for 2005 To understand the second half of the prize we need to learn more about two things:

3. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de One second is defined as the duration of 9 192 631 770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields. One second is defined as the duration of 9 192 631 770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields. F=4 F=3 Cs Precision Spectroscopy -To measure a frequency accurately. Definition of second: Originally defined in 1889 as the fraction 1/86 400 of the mean solar day. (not good because of irregularities in Earth’s rotation) Redefined in 1956 to 1/31 556 925.9747 of the length of the tropical year for 1900. (difficult to measure, long measurement times) Redefined in 1967 to an atomic reference. Some adavantages with atomic reference: 1. Fast oscillation (compared to the tropical year). 2. Identical atoms make it easy to ”copy” the reference. 3. Precise spectroscopic methods can be used to obtain time Some adavantages with atomic reference: 1. Fast oscillation (compared to the tropical year). 2. Identical atoms make it easy to ”copy” the reference. 3. Precise spectroscopic methods can be used to obtain time

4. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de 19 192 631 770234 1 second How accurately can the position of the peak be determined? Estimate: ~1/100th of the oscillation Direct counting gives an uncertainty of ~10 -10 Improved peak positioning gives ~10 -12 Interferometric techniques and signal averaging gives a final uncertainty of ~10 -15 Corresponds to a 1 second drift in 30 million years

5. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Fountain clocks Observatoire de Paris, NIST (Boulder), Observatoire Cantonal de Neuchâtel (Switzerland),… The NIST-F1 clock

6. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de For increased accuracy a faster oscillator is needed! 9.2 GHz MICROWAVE 9.2 GHz MICROWAVE Microwave electronics is highly developed technology! Optical oscillators! Green light: 532 nm corresponds to an oscillating frequency of 563000 GHz! Clock improvement with a factor 10 5 ! Unfortunately, no electronics can handle this speed...

7. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Laser-based precision spectroscopy Atoms can once more be used as references to stabilize the frequency of laser light. Tune the laser wavelength to an optical transition within an atom Laser frequencies have been stabilized to within <10 mHz. Comparing this to the frequency 500 THz one obtains a frequency uncertainty of < 10 -17 ! This would be a nice clock... But the problem remains: How to count the ”tick-tacks” of this clock? Solution: An optical frequency comb.

8. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Short laser pulses Laser pulses can be made extremely short, t < 10 femtoseconds. A short pulse has a broad spectrum. 2f rep f rep 3f rep The round-trip frequency is described by f rep A well-defined phase relation between the frequency components builds up the pulse

9. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de T=1/f rep   A phase shift between each pulse may appear due to a difference in group velocity and phase velocity. This depends on properties of the optical media etc. Temporal behavior of a pulse train

10. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Spectrum of a laser pulse Frequencies supported by the cavity: N f rep Pulse spectrum f n = n f rep +f 0, f 0 =  f rep /2  f 2n = 2n f rep +f 0 2f n - f 2n = 2(n f rep +f 0 ) - 2n f rep -f 0 = f 0 fnfn f 2n Tune the laser to remove f 0. This is done by ”self-reference”. Removes all dependenices of the laser, and generates a very uniform frequency comb. Need a full octave in the frequency spectrum (non-trivial)

11. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Reference laser Beat note with frequency similar to f rep can be measured Tune f rep to satisfy: f laser = N f rep This provides a direct link between the optical frequency and a radio frequency! Counting of a radio field is simple, and the stability is inherited from the optical clock... N is a rather large number ~10 5

12. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Comparing the spectrum of a reference laser with a frequency comb. f laser =N f rep -20-10010 20 2f laser f laser Count fringes! Convert two red photons into one blue photon. 2f red = f blue Frequency up-conversion

13. Heidelberg, 15 October 2005, Björn Hessmo, hessmo@physi.uni-heidelberg.de Summary Precision laser spectroscopy allows us to define optical frequency standards superior to the present Cesium time standard. The optical frequency comb allows us to transfer the stability of an optical reference to other frequencies where we can use electronics to handle signals. Is it time to redefine the second once more ? Time will tell...